The present invention relates to a surge-proof interface circuit such as is used in electronic devices such as, for example, in ballasts for lighting means. Via the interface circuit, data and/or control commands can be exchanged between a control unit for the lighting means and a ballast for lighting means.
Such interface circuits are generally frequently encountered where different electronic switching arrangements, for example via a bus system or signal lines, are connected. In this context, it may be the case that the electronic circuits coupled via the interface circuit operate with differently high operating voltages. In the case of ballasts for lighting means, for example, the ballast can have, on the one hand, a high operating voltage for operating the lighting means but, on the other hand, electronic circuit parts, for example for controlling the brightness of the lamp, can be connected to a bus or signal line system at which a relatively low voltage can be present in comparison with the operating voltage of the lighting means.
Nowadays, interface circuits are commonly used which can receive and process both digital signals such as DALI and line-voltage-oriented signals such as, for example, via a push button connected to the line voltage.
Since, during the installation of such illumination systems, mistakes may arise in the connecting lines or else overvoltages and voltage pulses on the bus system during operation, it would be desirable to use a surge-proof interface circuit having a high dielectric strength. For example, when operating a push button at the signal lines, connected to the line voltage, voltage pulses can arise at the interface circuit due to so-called key bounce or due to switching processes with relatively large load inductances, caused by transformers, chokes etc. which are coupled to the bus or signal lines. It is desirable, therefore, to create a surge-proof interface circuit so that the interface circuit will not be destroyed even in cases of inattention during the electrical installation or else due to other voltage pulses which may arise in the bus system.
An interface circuit having a fast overvoltage detector which is formed by a zener diode and a switching transistor is disclosed in patent specification DE 101 13 367 C1.
Other interface circuits according to the prior art frequently use a thyristor in order to secure dielectric resistance. FIG. 7 shows such an interface circuit having a thyristor in a circuit diagram.
The interface circuit 10 in FIG. 7 has connecting terminals 1 for signal lines or a bus system, respectively. The signal lines are coupled to the remaining interface circuit via a rectifier circuit which is formed by four diodes D1 to D4 in a Graetz circuit arrangement. The dielectric strength of the interface circuit 10 is secured via the thyristor X11. The thyristor X11 is connected in series with a transistor Q34. Furthermore, the interface circuit comprises optocouplers U7 and U8 and a constant-current source which is formed by the transistors Q2, Q20 and the resistors R31 and R34. The optocouplers U7 and U8 are coupled to a microprocessor, which is indicated by the symbol μC, so that data and control commands can be transmitted via the interface circuit 10 between the terminals 1 and the microprocessor which is connected at the points designated by μC. The optocoupler U7 controls in the return channel switch Q34 which is arranged in series with the thyristor X11. The switch Q34 is constructed as a low-voltage switch, whereas the thyristor, as already mentioned above, is intended to secure the dielectric resistance of the circuit. Transmission from the microprocessor to the terminals 1 via the interface circuit 10 occurs due to the fact that the switch Q34 is controlled via the optocoupler U7. When the switch Q34 is switched on, that is to say turned on, the potential at the cathode of thyristor X11 is pulled to a low potential. If then a voltage is present at terminals 1, the thyristor is turned on and the node 5 at the rectifier circuit is pulled in the direction of a ground potential. In other words, a low-resistance connection is established between the connecting terminals 1. As a result, a signal can be transmitted via the bus system when a voltage is present at terminals 1. Turning the thyristor X11 on takes place since a voltage is present at the gate, on the cathode side, of the thyristor over the path with respect to resistor R101 and R100, which voltage is higher than at the cathode of the thyristor.
If then, for example, the interface circuit 10 is switched to a line voltage, for example via a push button or switch, on sides of the terminals 1, key bounce can produce relatively narrow or short-time interference pulses up to the kilovolt range. These voltage pulses are called so-called signal sequence or burst pulses and, in the case of a corresponding dielectric strength of a circuit, this is called burst strength. The burst pulses are fast transient disturbance variables which are coupled in the power supply or in the signal inputs. These burst pulses can be distinguished by a short repetition rate and a low energy of the short-time disturbance. These burst pulses can easily also lead to a destruction of the interface circuit if no protective circuit such as, for example, a filter circuit is provided.
If, however, greater (load) inductances such as, e.g., transformers, chokes etc. are coupled to the terminals 1 via a control line and these are switched, high-energy overvoltage pulses or surge pulses having voltages also up to the kilovolt range can be produced. The surge pulses are high-energy transient voltage pulses which become noticeable as transient overvoltages or surge voltages such as can be produced during switching actions in the corresponding lines.
To filter or attenuate these burst or surge pulses, a filter can be connected ahead of an interface circuit. The filter can be dimensioned in such a manner that burst pulses of up to a voltage increase of some 10 volts are eliminated. However, this can be more difficult in the case of the surge pulses. Although these can be typically reduced greatly in the voltage amplitude by the filter, a 2 kV voltage pulse having an amplitude of, for example, 1 kV can nevertheless still reach the interface circuit. When a thyristor of the prior art is used, the thyristor can now be turned on wholly or partially by a steep edge of such a surge pulse since the thyristor is a positively coupled element. The “ignition” of the thyristor can then lead to the immediate destruction of the downstream transistor Q34 which, as a consequence of the current gain needed, can be a transistor of a low-voltage type with a dielectric collector-emitter strength of, for example, 45 to 80 volts. During this destruction process, the thyristor X11 is then finally turned on completely and can then also be destroyed. Although this turning-on or breakover of the thyristor can be influenced somewhat by circuit measures by connecting, for example, a capacitor directly from the gate to the cathode of the thyristor, the breakover can often not be adequately suppressed. The thyristor X11 is then in most cases destroyed not as a consequence of a high voltage but as a consequence of power overload.